Mets general manager Sandy Alderson confirmed Friday that Steven Matz will miss at least three weeks with a partial tear of his left lat muscle, but said the lefthander could be out longer.

"No one has guaranteed to us that it will be healed in three weeks," Alderson said during a news conference at Citi Field.

Alderson said Matz, a Stony Brook native, noticed the discomfort after his first start against Cincinnati on June 28 and was subsequently monitored by doctors, who determined that he could pitch in Los Angeles last Sunday.

After the start against the Dodgers, Alderson said it was decided that Matz would skip his next start, scheduled for Sunday against Arizona at Citi Field and Jonathon Niese would pitch.

An examination on Thursday revealed the partial tear. Matz will not throw for three weeks. He will be examined at that point and, if the healing has progressed to the point where the doctors are satisfied, "pass on to a different phase," Alderson said.

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"If the healing hasn't gotten to a point where we're satisfied, we'll probably be in a similar situation," Alderson said. "It's all a function of how his muscle responds to the next three weeks."

"It's definitely frustrating, but it could be worse," said Matz, a graduate of Ward Melville High School, who underwent Tommy John surgery in 2010. "It's a really minor thing. I'll get through it and be back. I'll try and stay positive."

Alderson, however, said he doesn't agree that it's a minor injury, but said it's not the worst injury that could've happened.

Matz was 2-0 with a 1.32 ERA since his promotion from Triple-A Las Vegas.